1. Field of the Invention
The present invention relates to a circulating body that is used for a fixing device fixing an unfixed image under heating and pressure in an image forming device such as copiers, printers and facsimile machines and is driven in such a manner that its surface is circulated along a fixed route and also to a fixing device using the circulating body.
2. Background Art
It is usually necessary to form a permanent image by fixing an unfixed toner image formed on a recording sheet in copiers and the like making use of an electrophotographic process. As the fixing method, a solvent fixing method, pressure fixing method and heat fixing method are known. Among these fixing methods, the solvent fixing method has the drawback that solvent vapor is emitted during fixing, giving rise to many problems concerning odors and sanitation. The pressure fixing method has the drawback that it is inferior in fixing characteristics to other fixing methods. Accordingly, the solvent fixing method and the pressure fixing method are both limited in the range of practical fields and therefore, the heat fixing method in which a toner is melted under heating to fuse it on a recording sheet (image-receiving material) is widely used.
As a device used in such a heat fixing method, a heat fixing device is known which has a heat roll (also called a fixing roll) provided with a heater lamp inside of a cylindrical core bar and a heat resistant releasable layer formed on the outside peripheral surface of the core bar and a pressure roll disposed in pressure contact with the heat roll and having a structure in which a heat resistant elastic body layer is formed on the outside peripheral surface of a cylindrical core bar. In the heat fixing device having this structure, a pressure of 1 to 15 kg/cm2 (preferably 3 to 10 kg/cm2) is applied to the heat roll and the pressure roll from each other and a recording sheet such as a common paper on which an unfixed toner image is formed is inserted and passed through between both rolls to thereby fix, and fixing using a heat fixing roll system is thus carried out. In the heat fixing device as aforementioned, the surface of the pressure roll is moved circularly on a fixed route and this pressure roll is a sort of the aforementioned circulating body.
The heat fixing device (heat roll type fixing device) adopting this heat fixing roll system has higher heat efficiency than devices using other fixing systems such as a hot air fixing system and an oven fixing system and therefore, the heat fixing device requires low power, is superior in high speed performance and is reduced in danger of a fire caused by paper jamming. This heat fixing device is therefore most widely used.
However, more energy saving and more promotion of fixing speed are both desirably accomplished in recent heat fixing devices. In order to satisfy this, it is necessary to increase the width of a nip zone, namely a nip width, that actually sandwiches a recording sheet when fixing under heating. Under this situation, a belt nip type fixing device has been recently developed to solve these problems.
As the belt nip type fixing device, for instance, a device is known which is provided with a rotating heat fixing roll with a built-in heating source, a resin film tubular body that is in pressure contact with the heat fixing roll and is moved with rotating together with the heat fixing roll and a pressure pad that is disposed inside of the resin film tubular body and presses the resin film tubular body against the heat fixing roll to form a nip section between the resin film tubular body and the heat fixing roll and fixes an unfixed toner image to a recording sheet by passing the recording sheet through the nip section (see, for example, Japanese Patent Application Laid-open No. 11-24457). In such a belt nip type fixing device, the surface of the resin film tubular body is moved circularly on a fixed route and is therefore also a sort of the aforementioned circulating body.
As the circulating bodies such as the resin film tubular body and the pressure roll, those having a layer of a copolymer (hereinafter abbreviated as PFA if necessary) of tetrafluoroethylene and perfluoroalkyl vinyl ether on the surface and those having a rubber type surface are used to obtain releasability from a recording sheet and a toner image.
Also, in the belt nip type fixing device, a lubricating material such as a lubricant or a low-friction film is interposed between the pressure pad and the resin film tubular body to thereby reduce sliding resistance between the resin film tubular body and the fixed pressure pad.
However, the belt nip type fixing device has the problem that when the lubricating material is thermally deteriorated or worn with working time, the sliding resistance between the resin film tubular body and the fixed pressure pad is raised, with the result that the running of the recording sheet becomes unstable, leading to occurrences of paper wrinkles and image defects.
Such a problem is a particular one regarded as important in the belt nip type fixing device. However, this problem is not exclusive in the belt nip type fixing device but possibly arises likewise even in the aforementioned heat roll type fixing device in the case where the resistance of the pressure roll (circulating body) is increased when its surface is circulated.